Serialize startup chain source requests

When concurrent requests share an Electrum client connection, a clean
EOF can cause the active reader to return an error without waking other
requests waiting on the connection. Those requests remain blocked in
spawn_blocking, causing Tokio runtime shutdown to wait indefinitely
for them to finish.

As a temporary fix, we run the startup fee update and zero-fee
commitment validation in sequence. This avoids concurrent requests on
the shared Electrum connection leaving a blocking task stranded after
connection closure.

Co-Authored-By: HAL 9000

Author: tankyleo

src/lib.rs

@@ -347,12 +347,12 @@ impl Node {
 		// TODO: drop 0FC chain source validation when support is ubiquitous
 		let chain_source = Arc::clone(&self.chain_source);
 		self.runtime.block_on(async move {
-			tokio::try_join!(
-				chain_source.update_fee_rate_estimates(),
-				chain_source.validate_zero_fee_commitments_support_if_required(
-					zero_fee_commitments_support_required
+			chain_source.update_fee_rate_estimates().await?;
+			chain_source
+				.validate_zero_fee_commitments_support_if_required(
+					zero_fee_commitments_support_required,
 				)
-			)
+				.await
 		})?;
 
 		// Bind listeners before spawning background tasks so startup errors cannot leave loops